This application claims the priority of Korean Patent Application No. 10-2004-0090918, filed on Nov. 9, 2004 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to methods and apparatuses for removing noise from digital images, and, more particularly, to a method and apparatus for removing hot pixels from a digital image, in which an imaging device is physically moved between successive image frame captures, and the resulting image frames are median combined.
2. Description of the Related Art
Digital imaging devices such as digital cameras and digital camcorders use charged coupled devices (CCDs) in place of film. A CCD is a sensor that converts light into electric signals. It has a silicon chip with minute photodiodes disposed on top of it. These photodiodes are organized into numerous “pixels” on top of the silicon chip in a checkered pattern. Each pixel of the CCD captures light that is eventually converted into one “pixel” of an image.
CCDs are sensitive to heat. As a result, heat noise is frequently generated when a CCD operates, which prevents a cell from accurately generating the color for its pixel. Pixels generated by CCDs as a result of this heat noise are called hot pixels. Hot pixels usually show up at a fixed location for a short period of time. That is why hot pixels are also called “fixed pattern noise.” Fixed pattern noise frequently occurs when the shutter of a digital camera is open for a long period of time, and it hinders the smooth reproduction of an image.
FIG. 1 is a flow chart illustrating a conventional method of removing fixed pattern noise. First, an image frame (Frame 1) is photographed according to a set sensitivity (ISO) and exposure time (100). The image frame (Frame 1), including the fixed pattern noise, is stored in a memory (not shown) (102). Then, a so-called “dark frame” (Frame 2) is photographed using the same ISO and exposure time as the photographed image frame (Frame 1) (104). A dark frame (such as Frame 2) is an image photographed with the shutter manually closed to physically block all light from reaching the CCD. The dark frame (Frame 2) is not affected by light but does include the fixed pattern noise.
The same exposure time is used when photographing the image frame (Frame 1) and the dark frame (Frame 2), so as to accurately reproduce the fixed pattern noise of in the image frame (Frame 1) on the dark frame (Frame 2). Ideally, the same number n hot pixels will occur on both the image frame (Frame 1) and the dark frame (Frame 2).
The photographed dark frame (Frame 2) is then stored in the memory (106). Frame subtraction is then performed to remove the fixed pattern noise. That is, the dark frame (Frame 2) is subtracted from the image frame (Frame 1) (108). Through such frame subtraction, a new image frame (Frame 3) with the fixed pattern noise removed is generated. The new image frame (Frame 3) is then corrected by image processing, and an image file (e.g., JPEG) is generated (110).
The method illustrated in FIG. 1 the total photographing time is more than twice the set exposure time of the digital image processing apparatus. For example, referring to FIG. 9A (which illustrates the total photographing time of a conventional digital image processing apparatus) if the exposure time is 10 seconds, {circle around (1)} the image frame (Frame 1) is photographed for 10 seconds, {circle around (2)} the dark frame (Frame 2) is photographed for 10 seconds, {circle around (3)} the noise removal after frame subtraction takes s1 seconds, and {circle around (4)} the post-processing of the frame subtraction takes s2 seconds. Consequently, the total photographing time ({circle around (1)}+{circle around (2)}+{circle around (3)}+{circle around (4)}) is more than twice the exposure time, that is, more than 20 seconds.
Thus, it can be seen that there is a need for a new method and apparatus for removing noise from a digital image that avoids the foregoing problems.